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The cell envelope of  Mycobacterium tuberculosis  ( M.tuberculosis ) is composed of a variety of lipids including mycolic acids, sulpholipids, lipoarabinomannans, etc., which impart rigidity crucial for its survival and pathogenesis. Acyl CoA carboxylase (ACC) provides malonyl-CoA and methylmalonyl-CoA, committed precursors for fatty acid and essential for mycolic acid synthesis respectively. Biotin Protein Ligase (BPL/BirA) activates apo-biotin carboxyl carrier protein (BCCP) by biotinylating it to an active holo-BCCP. A minimal peptide (Schatz), an efficient substrate for  Escherichia coli  BirA, failed to serve as substrate for  M. tuberculosis  Biotin Protein Ligase ( Mt BPL).  Mt BPL specifically biotinylates homologous BCCP domain,  Mt BCCP 87 , but not  Ec BCCP 87 . This is a unique feature of  Mt BPL as  Ec BirA lacks such a stringent substrate specificity. This feature is also reflected in the lack of self/promiscuous biotinylation by  Mt BPL. The N-terminus/HTH domain of  Ec BirA has the self-biotinable lysine residue that is inhibited in the presence of Schatz peptide, a peptide designed to act as a universal acceptor for  Ec BirA. This suggests that when biotin is limiting,  Ec BirA preferentially catalyzes, biotinylation of BCCP over self-biotinylation. R118G mutant of  Ec BirA showed enhanced self and promiscuous biotinylation but its homologue, R69A  Mt BPL did not exhibit these properties. The catalytic domain of  Mt BPL was characterized further by limited proteolysis. Holo- Mt BPL is protected from proteolysis by biotinyl-5′ AMP, an intermediate of  Mt BPL catalyzed reaction. In contrast, apo- Mt BPL is completely digested by trypsin within 20 min of co-incubation. Substrate selectivity and inability to promote self biotinylation are exquisite features of  Mt BPL and are a consequence of the unique molecular mechanism of an enzyme adapted for the high turnover of fatty acid biosynthesis.

Diversity in Functional Organization of Class I and Class II Biotin Protein Ligase